1. Field of the Invention
This invention relates to a brushless motor drive circuit.
2. Background Prior Art
Brushless motor drive circuits of so-called current drive type are widely known and popularly used. Since brushless motor drive circuits of the current drive type are accompanied by the problem of generation of torque ripples in the current level, a drive circuit as illustrated in FIG. 8 of the accompanying drawings has been proposed to reduce such torque ripples (Japanese Patent Laid Open Publication No. 61-42288).
FIG. 8 illustrates in block diagram form a prior art arrangement which is of a 3-phase 120.degree. soft switching type comprising three position detecting means. FIGS. 2(I) and (II) illustrate respectively the waveform of a signal detected by the position detecting means and that of a signal processed by the signal processing means.
In FIG. 8 a brushless motor drive circuit comprises at least a stator (not shown), a rotor (not shown), Hall devices 11u, 11v and 11w which serve as position detecting means, signal processing means 20 comprising Hall amplifying circuits 16u, 16v and 16w and a signal composing circuit 12d, a group of positive side switching devices 13a, a group of negative side switching devices 13b, a current detecting resistor 14 that constitutes current detecting means and current control means 15.
These components are described below.
The stator of the prior art has m(=3 hereinafter)-phase drive coils Lu, Lv and Lw as illustrated in FIG. 1. The rotor, on the other hand, has magnetic poles (not shown). The Hall devices 11u, 11v and 11w generate and transmit signals Vu Vv and Vw in the form of 3-phase sine waves in accordance with the positional relationship between the stator and the rotor as illustrated in FIG. 2(I) and the output signals are respectively supplied to the Hall amplifying circuits 16u, 16v and 16w. The signals which are logarithmically compressed in the Hall amplifying circuits 16u, 16v and 16w are then supplied to the signal composing circuit 12. The signal processing means 20 comprising the Hall amplifying circuits 16u, 16v and 16w and the signal composing circuit 12 have the role of transforming the output signals Vu, Vv and Vw from the Hall devices 11u, 11v and 11w into rectangular wave pulse signals by flattening the inflection points so as to develop 3-phase soft switching signals from them as illustrated in FIG. 2(II) The output signal of signal composing circuit 12 is then supplied to signal circuits 17a and 17b. These signal circuits 17a and 17b provide respectively signals Sda and Sdb by appropriately adjusting the amplitude of the 3-phase soft switching signals in accordance with current error signal Sc. The signal Sda from the signal circuit 17a is supplied to the positive side group of switching devices 13a, whereas the signal Sdb from the signal circuit 17b is transmitted to the negative side group of switching devices 13b. The positive side group of switching devices 13a switches the current supplied to the 3-phase drive coils Lu, Lv and Lw in accordance with the signal Sda transmitted from the signal composing circuit 12 by way of the signal circuit 17a. On the other hand, the negative side group of switching devices 13b switch the current supplied to the 3-phase drive coils Lu, Lv and Lw in accordance with the signal Sdb transmitted from the signal composing circuit 12 by way of the signal circuit 17b. The positive side group of switching devices 13a of this prior art comprises transistors Q.sub.31 through Q.sub.33. The negative side group of switching devices 13b, on the other hand, comprises transistors Q.sub.34 through Q.sub.36.
The current detecting resistor 14 can detect currents flowing through the drive coils Lu, Lv and Lw. The current control means 15 is designed so as to be capable of controlling the level of the currents being supplied to the drive coils Lu, Lv and Lw from the positive and negative side groups of switching devices 13a and 13b and typically comprises a current feedback amplifier.
A brushless motor drive circuit having a configuration as described above operates in the following manner.
The Hall devices 11u, 11v and 11w generate respectively output signals Vu, Vv and Vw in the form of a 3-phase sine wave as illustrated in FIG. 2(I) in response to the positional relationship between the stator and the rotor. The output signals Vu, Vv and Vw from the Hall devices 11u, 11v and 11w are then transmitted to the Hall amplifying circuits 16u, 16v and 16w. The signals coming from the Hall amplifying circuits 16u, 16v and 16w are supplied to the signal composing circuit 12, where the output signals Vu, Vv and Vw of the Hall devices are transformed into rectangular pulse signals as the inflection points are flattened by means of logarithmic compression and 3-phase soft switching signals (V.sub.u ', V.sub.v ' and V.sub.w '), as illustrated in FIG. 2(II), are developed from them. The composed 3-phase soft switching signals are then used to produce a positive side signal Sda since as their amplitudes are independently modified in accordance with the current error signal Sc in the signal circuit 17a. Similarly, they are used to produce a negative side signal Sdb since their amplitudes are independently modified in accordance with the current error signal Sc in the signal circuit 17b. The positive side signal Sda from the signal circuit 17a and the negative side signal Sdb from the signal circuit 17b are then supplied respectively to the positive side group of switching devices 13a and the negative side group of switching devices 13b for switching the current supplied to the 3-phase drive coils Lu, Lv and Lw. The currents flowing through the drive coils Lu, Lv and Lw are detected by the current detecting resistor 14.
Current control means 15 compares the detection signal from the current detecting resistor 14 and a predetermined reference level V.sub.CTL of the currents to be supplied to the drive coils Lu, Lv and Lw and forms a current error signal Sc for controlling the level of the currents supplied to the drive coils Lu, Lv and Lw. The current error signal Sc is supplied to the signal circuits 17a and 17b.
FIG. 4, illustrates the waveform of the current .alpha.I'u and that of .alpha.Iu having phase U indicated in FIG. 8. (Note that the currents with phases V and W have a waveform similar to the one illustrated but their phases are shifted by 120.degree. and 240.degree. respectively.) As seen from the waveform of FIG. 4, there exist timings when no currents are supplied for a predetermined period of time or when a portion of the circuit becomes open (as indicated by t.sub.0 in FIG. 4). The existence of timings for providing openness of a portion of the circuit can also be verified for phase V and phase W. Since the supply of current to the 3-phase drive coils Lu, Lv and Lw is sequentially switched in accordance with the output signals of the Hall devices 11u, 11v and 11w, which serrve as rotor angular position detecting means, the rotor is forced to rotate.
Meanwhile, the currents that pass through the 3-phase drive coils Lu, Lv and Lw are collected in the current detecting resistor 14 and converted to voltage E.sub.1. The voltage E.sub.1 is then used as a current detection signal which is supplied to the current control means 15. The current control means 15 compares voltage E.sub.1 and a reference current level V.sub.CTL and controls the variable current source circuit I.sub.CTL so that the voltage E.sub.1 is always kept equal to the reference current level V.sub.CTL. As long as the reference current level V.sub.CTL is unchanged, the voltage E.sub.1 generated by the current detecting resistor 14 is maintained to be constant so that constant currents are supplied to the drive coils Lu, Lv and Lw for driving the brushless motor.
If it is assumed that the 3-phase counter electromotive voltages generated in the drive coils Lu, Lv and Lw have a sine waveform, the torques Tu, Tv and Tw generated in the phases are respectively expressed by formulas shown below. EQU Tu.varies.(.alpha.I'u-.alpha.Iu)sin(.theta.-30.degree.) EQU Tv.varies.(.alpha.I'u-.alpha.Iv)sin(.theta.-90.degree.) EQU Tw.varies.(.alpha.I'u-.alpha.Iw)sin(.theta.-210.degree.)
Thus, composite torque T is expressed by EQU T=Tu+Tv+Tw
FIG. 4 shows the composite torque T, which entails torque ripples of approximately 14.3%. The voltages V.sub.u ', V.sub.v ' and V.sub.w ' produced by the signal composing circuit 12 are expressed respectively by the formulas (4) through (6) and the amplitudes of the voltages V.sub.u ', V.sub.v ' and V.sub.w ' are equal to 400 m volts, p--p.
A motor of the current drive type is inevitably accompanied by the problem of torque ripples that take place with a frequency equal to the least common multiple of the number of poles of the stator drive coil and that of the rotor magnet at a level which is approximately 14% of the overall torque.
In order to overcome the problem of torque ripple, previously an additional torque ripple reduction circuit was required. Brushless motor drive circuits that can reduce the level of torque ripples are a1so disclosed in Japanese Patent Publication No. 56-34551 and Japanese Patent Disclosure Nos. 58-192490, 59-35585 and 59-76192.
While brushless motor drive circuits such as described above are realized in the form of integrated circuits (ICs), additional torque ripple reduction circuitry can increase the number of elements of the ICs and consequently make them complicated and large.
Besides, since any known brushless motor drive circuits utilize the waveforms of output signals of a Hall device which are used as means for detecting the angular position of the rotor to form torque ripple reduction signals, the effect of reduction of torque ripples can be adversely affected by uneven waveforms of the output signals of the Hall device, any distortion of the device and its temperature-related characteristics. In short, the torque ripple reduction effect of such circuits is dependent on the precision with which the Hall devices they comprise are made.
It is therefore the object of the present invention to provide a brushless motor drive circuit that does not require any complicated circuit configuration and is not affected by the composition of the torque ripple detecting means it comprises in forming torque ripple reduction signals.
In accordance with the invention, a brushless motor drive circuit comprises a stator having m-phase drive coils, a rotor having a number of magnetic poles, a plurality of angular position detecting means for providing output signals in the form of m-phase sine waves representing the positional relationship between the stator and rotor, signal processing means for transforming the output signal from each position detecting means into a rectangular wave pulse signal by flattening the inflection points of the sine wave by means of logarithmic compression so as to compose m-phase soft switching signals, a group of switching devices arranged in positive and negative sides for switching the current supplied to the m-phase drive coils in response to the output signal from the signal processing means, current control means for controlling the level of the current supplied to the m-phase drive coils by means of the positive and negative sides switching device and current detecting means arranged to detect the level of the current flowing through the drive coils and, by the use of negative feedback, applying the detecting current level to the current control means, reactive current being supplied to the current detecting means for reduction of torque ripple.
Also in accordance with the invention, a brushless motor drive circuit comprises a stator having an m-phase drive coil, a rotor for being rotationally driven by the stator, position detecting means for providing an output signal in the form of an m-phase sine wave representing the positional relationship between the rotor and stator, a signal composing circuit for transforming the output signal into a rectangular pulse signal by flattening the inflection points of the sine wave by means of logarithmic compression, a plurality of switching elements for switching the electric current supplied to the m-phase drive coil in response to the output signal of the signal composing circuit, the switching elements having an ON and OFF condition, current control means for controlling the rate of the current supplied to the m-phase drive coil by the plurality of switching elements, resistive means for detecting electric current flowing through the drive coil and for providing a negative feedback signal to the current control means, simultaneous ON-section generating means for simultaneously causing those switching elements having the same phase to be in an ON condition in order to supply the resistive means with the reactive current during the periods of current supply when no current is to be supplied to the m-phase drive coil and reactive current braking means for braking reactive current during the periods of electric current when current is to be supplied to the m-phase drive coil.
For a better understanding of the present invention, reference is made to the following description and accompanying drawings while the scope of the invention will be pointed out in the appended claims.